Remove no-effect @skipIf on test_untracked_files

It looked like test_untracked_files was sometimes skipped, and
specifically that it would be skipped on Cygwin. But the `@skipIf`
on it had the condition:

    HIDE_WINDOWS_KNOWN_ERRORS and Git.is_cygwin()

HIDE_WINDOWS_KNOWN_ERRORS can only ever be true if it is set to a
truthy value directly (not an intended use as it's a "constant"),
or on native Windows systems: no matter how the environment
variable related to it is set, it's only checked if is_win, which
is set by checking os.name, which is only "nt" on native Windows
systems, not Cygwin.

So whenever HIDE_WINDOWS_KNOWN_ERRORS is true Git.is_cygwin() will
be false. Thus this condition is never true and the test was never
being skipped anyway: it was running and passing on Cygwin.

test/test_repo.py

@@ -13,7 +13,7 @@
 import pickle
 import sys
 import tempfile
-from unittest import mock, skipIf, SkipTest, skip
+from unittest import mock, SkipTest, skip
 
 import pytest
 
@@ -42,7 +42,7 @@
 )
 from git.repo.fun import touch
 from test.lib import TestBase, with_rw_repo, fixture
-from git.util import HIDE_WINDOWS_KNOWN_ERRORS, cygpath
+from git.util import cygpath
 from test.lib import with_rw_directory
 from git.util import join_path_native, rmtree, rmfile, bin_to_hex
 
@@ -764,16 +764,6 @@ def test_blame_accepts_rev_opts(self, git):
         self.rorepo.blame("HEAD", "README.md", rev_opts=["-M", "-C", "-C"])
         git.assert_called_once_with(*expected_args, **boilerplate_kwargs)
 
-    @skipIf(
-        HIDE_WINDOWS_KNOWN_ERRORS and Git.is_cygwin(),
-        """FIXME: File "C:\\projects\\gitpython\\git\\cmd.py", line 671, in execute
-                    raise GitCommandError(command, status, stderr_value, stdout_value)
-                GitCommandError: Cmd('git') failed due to: exit code(128)
-                  cmdline: git add 1__��ava verb��ten 1_test _myfile 1_test_other_file
-                          1_��ava-----verb��ten
-                  stderr: 'fatal: pathspec '"1__çava verböten"' did not match any files'
-                """,
-    )
     @with_rw_repo("HEAD", bare=False)
     def test_untracked_files(self, rwrepo):
         for run, repo_add in enumerate((rwrepo.index.add, rwrepo.git.add)):